The present invention is directed to a data processing system, and more particularly, to a light pen detection system in which data characters displayed on the screen of a cathode ray tube are sensed by the positioning of a light pen adjacent a selected data character, which data character is utilized in processing data for display on the cathode ray tube or for use in a data terminal device. In the operation of the system, data in the form of alphanumeric characters are displayed on the screen of a cathode ray tube. A light pen connected to a processor located in a terminal device is positioned adjacent the character or a field of characters desired to be read. After the operator is sure that the light pen is properly pointed at the desired character or field, a switch is activated. Signals generated by the light pen as a result of sensing the display of the selected character is transmitted to control circuits which determine the position of the character sensed by the light pen. The character position sensed is then transmitted to a processor which utilizes this information, to provide further processing operations with respect to the character sensed at that position. One problem encountered in this type of sensing operation is that of verifying to the operator that the character desired to be sensed is in fact the character detected by the light pen since there is no visual verification of the exact location of the light pen in relation to the desired character due to parallax and also to the inherent difficulty in selecting one character out of a field of characters.
This problem has been addressed by the prior art. In U.S. patent to Anderson et al., U.S. Pat. No. 4,017,680, the brightness of the character sensed by the light pen is increased. In U.S. patent to Gundrum, U.S. Pat. No. 3,509,350, a portion of the character sensed by the light pen is distorted, by leaving the portion blank as displayed on the screen of the CRT. Both these systems require a proper relationship between the contrast of the character displayed and the background which may not be readily apparent to the operator, thus leading the operator to miss the verifications of the character sensed. In U.S. patent to Iwamura et al., U.S. Pat. No. 3,997,891, position markers are displayed on the character sensed by the light pen to overcome this problem. Again, the operator can easily overlook these markers, resulting in the prolonging of the sensing operation which becomes bothersome to the operator and therefore leads to an erroneous operation of the system.
A second problem encountered in light pen operations occurs when placing the light pen in the middle of two characters or two lines of characters when attempting to read a character, wherein the light pen tends to pick up light rays from both characters or lines of characters, resulting in a mis-operation. Agains, this problem is the result of parallax, since it is difficult for the operator to locate the light pen in the middle of the character desired to be read due to the smallness of the character and the crowded condition when located in a field.
It is therefore the principal object of this invention to provide a light pen detection system which will provide an obvious indication of the character sensed by the light pen. It is a further object of this invention to provide a light pen detection system which will sense only one character at a time. It is a further object of this invention to provide a light pen detection system which will accomplish the previous cited objects in a low-cost manner.